Subsidence control of generally empty spatial volumes below the earth's surface, i.e., subterranean voids, such as abandoned underground mines or tunnels, is a continuing problem especially in regions where subsequent surface development above the mine or tunnel has taken place. Most abandoned mines are normally accessible only through boreholes drilled from the surface into the mine cavity Common methods currently used for control of subsidence and stabilization of such underground mines include slurry backfilling, pneumatic backfilling, or cement grouting backfilling techniques.
Slurry backfilling has some advantages in that it is a generally low cost technique which is helpful for large scale projects where gravel or other mine refuse is readily available at or sufficiently close to the site which is to be backfilled. However, slurry backfilling technique often tend to cause further subsidence at some project sites and the technique has generally been avoided by those in the art for several years. An example of a slurry backfilling approach is disclosed in U.S. Pat. No. 3,817,039, issued to J. D. Stewart et al on June 18, 1974.
Cement grouting techniques normally use concrete columns which are formed by pumping concrete through a plurality of boreholes so as to provide support under surface structures. While such technique provides reasonably good support it is highly costly and is normally used on a relatively limited basis where its high cost can be justified.
A pneumatic, or dry, injection technique can be an effective method of backfilling underground mines with dry fill material if the subterranean cavity itself is not already flooded with water. Dry injection is often necessary to prevent the ground water contamination which is caused by using slurry methods. Further dry injection methods are used instead of slurry backfill techniques because the latter may tend to cause additional subsidence due to the flow of the water and fill material combination into an otherwise dry mine. Even though dry injection techniques tend to be higher in cost per ton than slurry injection methods, they can be economically feasible if the mine void volume is relatively small since no water wells are needed and no large pipe network is required. Accordingly, dry injection methods are often the more desired methods of the art when the subterranean void is relatively dry.
A commonly used dry injection technique is similar to the pump/slurry method disclosed in the aforesaid Stewart et al patent, except that air is used as the conveying fluid instead of water. Air at a relatively low pressure and high volume is blown into a gravel, or other fill material, metering device, such as a pocket feeder or lockhopper device. The fill material, e.g. sand, gravel, or a combination thereof, or some other solid fill material, is conveyed into a lockhopper via a conveyor belt or similar conveying device. The lockhopper feeds the fill material into an airstream conduit, the air and fill material passing vertically through the conduit to the mine void in a manner such as is shown in FIG. 1. Such a method raises at least two major problems.
First, the material is not effectively directed into the mine at the entry point thereof by any deflection, or flow turning, device, i.e. the flow material is merely directed downwardly in a vertical direction from the conduit exit and the fill material does not extend very far beyond the bottom of the conduit. Secondly, the pocket feeder or lockhopper is subject to extensive wear, especially at the seal faces which must seal the air and the abrasive fill material.
Typically, the fill materials which have been most often used for such dry injection method are flyash, sand or gravel, or other similar materials. While flyash tends to work reasonably well in that it flows relatively freely in the mine void so as to allow relatively large amounts of material to be injected at each borehole, such material is usually not utilized because it is not readily available and is relatively costly. Sand and/or gravel are generally more readily available and more economical to use. When the latter material is transported pneumatically, it can be moved relatively easily down the borehole pipe but, as it enters the mine void through the vertical borehole, it builds up a conical pile of material beneath the borehole as shown in FIG. 1. The conical pile assumes a general angle of repose and then builds up to a height where it reaches the bottom of the borehole pipe so that the flow is thereupon choked off and no further fill material can enter the void.
Various methods have been tried in order to mechanically direct the downward flow away from the exit end of the borehole pipe so that the direction of travel of the fill material is deflected away from the vertical. For example, elbows or other baffles or deflection surfaces are positioned at or in the bottom of the borehole pipe but such devices achieve little or no success when used with abrasive materials such as sand and/or gravel, and the like. The high velocity airflow at the pipe exit, e.g., up to 140 feet per second, tends to cause the abrasive fill material to bombard whatever deflection or turning device is installed at the end of the borehole pipe and, thereby, bring about rapid wear thereof, and subsequent failure of the device.
As a result of the problems which have arisen with the injection of dry solid fill materials, an alternative solution currently used in the field is to drill a plurality of boreholes at strategic locations over the subterranean void and to blow solid material straight down through each borehole without using any deflection devices. Such an approach further unduly increases the cost and time needed to back fill the subterranean void.
It is desirable to devise a technique for performing such backfill operation at a reasonable cost and avoid the problems which arise in using the above discussed techniques.